Can I resume retopologizing a mesh in Retopoflow after exiting the tool?
I ran into this problem as well. You have to be in edit mode to re-enter Retopoflow.
With the RetopoFlow mesh selected in edit mode, press the diamond button to start RetopoFlow or open the menu and pick any of the RF options.

So, thankfully there is no need to retopologize all in one go.